1. Field of the Invention
The invention relates to a circuit arrangement for bidirectional operation of sound transducers disposed at ends of a measuring section, having a signal generator and a receiver which are connected to the sound transducers by means of switching elements.
2. Description of the Prior Art
A circuit arrangement for sound tranducers is disclosed in De 198 10 798 A1. The known circuit arrangement comprises a signal generator which has a transmission output stage connected downstream of it. The transmission signal generated by the signal generator is optionally passed to one of the two ultrasonic transducers with the aid of a multiplexer. The multiplexer can connect the ultrasonic transducers to a preamplifier, which is used as a receiver and has an evaluation circuit connected downstream of it. The multiplexers are optionally used to connect one of the two ultrasonic transducers to the signal generator, whereas the respective other ultrasonic transducer is connected to the preamplifier and to the evaluation unit. Accordingly, ultrasonic signals can be transmitted in both directions along the measuring section defined by the ultrasonic transducer. The measuring section crosses a gas mass flow such that different propagation times for the sound signal result for the two transmitting directions. The velocity of the gas flow can then be inferred from the different propagation times.
Flowmeters for liquid or gaseous media can be set up with the aid of the known circuit arrangement. This includes metering of fuels, filling of drinks and calculations of the consumption of water or gas.
The flow measurement apparatuses set up with the circuit arrangement have a number of advantages. One advantage is that the apparatuses are suitable for all liquids and gases. Furthermore, the medium itself is not distorted by the measurement. Since the apparatuses manage without mechanical parts which are subject to wear, the apparatuses have a high long-term stability. Furthermore, high temporal resolutions are possible since the propagation times of the ultrasonic signals are generally less than one millisecond. Another advantage is that the direction of the flowing medium can be detected and that the apparatus has a linear measurement characteristic. Finally, additional information relating to substance properties or substance compositions can also be measured with the aid of the measuring apparatuses by determining, for example, the sound velocity of the ultrasonic signals in the medium. The low space requirement and the comparatively small mass of the measuring apparatus as well as the low power requirement, which enables use together with current-saving microcontrollers, should finally be emphasized.
So that no measurement errors occur in the known circuit arrangement, reciprocity must be ensured in the transmitting and receiving paths. For this purpose, use is respectively made of a signal generator and a receiver which is connected to the respective ultrasonic transducers. Furthermore, identical terminating impedances are connected in series with the ultrasonic transducers, said impedances ensuring that the differences in the electrical properties of the sound transducers, which differences arise even when the ultrasonic transducers are selected in pairs, do not result in zero point deviations during measurements on the stationary medium or in characteristic curve distortions.
For optimal power matching, the impedance of the terminating impedances must be selected to be equal to the impedance of the ultrasonic transducers at the resonant frequency of the ultrasonic transducers. Half of the signal emitted by the signal generator is thus lost at the terminating impedances. In the known circuit arrangement, the signal generator must therefore provide high signal voltages so that an ultrasonic signal with a sufficient signal strength arrives at the receiving ultrasonic transducer. The high signal voltages to be supplied by the signal generator may result in interference between the transmitting path and the receiving path, which interference distorts the measurement of the flow velocity. In addition, further devices in the vicinity may be subject to interference.